The present invention generally relates to the field of flow sensors, and more particularly to flow sensors based on the use of acoustic waves.
The measurement and control of fluid flows is fundamental to many scientific and technological areas. Numerous methods for measuring flow are available. These include a number of mechanical devices such as variable area flow meters, pitot tubes, and turbine meters; a variety of ultrasonic flowmeters; and laser doppler shift devices. The most commonly used flow measurement device today is the hot wire anemometer (HWA). The HWA consists of a small, electrically heated element exposed to the fluid for the purpose of measuring fluid velocity. Flow of fluid carries heat away by convection. This results in a change in the temperature and hence in the resistance of the element. The change in resistance is usually measured by using the element as one of the arms of a Wheatstone bridge. The measured offset voltage of the bridge is proportional to fluid velocity. Although the theoretical aspects of HWA's have advanced significantly, practical difficulties in using these devices continue. The problems include those associated with the electronics, probe stability, calibration, and frequent breakage of the fragile filament. Also, the device is not suitable for large scale mass production. The hot film anemometer (HFA) is another commonly used flow measurement device which employs the same principle as the HWA. Many of the problems associated with the HWA also occur in the hot film anemometer. In recent years there has been a growing interest in developing highly sensitive, miniature flow sensors based on semiconductor and other solid state phenomena. Examples of such sensors reported in the literature include a lithium tantalate pyroelectric anemometer and a variety of silicon based devices.
In general, commercially available flow sensors for air and other gases commonly use large components, typically in a configuration with a flow tube having a heated section and adjacent upstream and downstream temperature detector sections. Such instruments use relatively large flow rates, require large power inputs, and are not small enough and sufficiently sensitive for many applications of interest. A need exists for a flow sensor that is small in size, consumes low power, has high sensitivity, and fast response. In addition, the device must be mass producible on a large scale and be low in cost. The literature contains several examples of attempts to improve the flow sensing art with respect to these needs. As previously noted, a number of flow sensors using silicon and its semiconducting properties have been reported, and the use of a pyroelectric material to measure flow has also been considered.
The present invention is concerned with the use of acoustic wave devices for measuring gas flow. The discussion will mainly deal with the use of surface acoustic waves (SAW's). However, the basic techniques described here can also be used with other types of acoustic waves such as plate waves or Lamb waves, surface skimming bulk waves (SSBW), reflected bulk waves (RBW), etc. The basic device used is a delay-line or resonator stabilized SAW oscillator which is heated to a suitable temperature above ambient. Flow of gas over the device carries heat away by convection. This lowers the substrate temperature thereby causing change in the oscillator frequency.
Two previous reports on the use of surface acoustic waves to measure gas flow are known. The first is an article by Nisar Ahmad, presented at the 1985 IEEE Ultrasonics Symposium, Oct. 1985 entitled "Surface Acoustic Wave Flow Sensor." The Ahmad device, however, has the following disadvantages: (1) the heater is situated outside the delay line resulting in poor heating; (2) the size of the device is increased because of the area needed for the heater; and (3) the SAW delay line used by Ahmad can support several modes of oscillation which can cause ambiguity in the output of the device. Additionally, the response of Nisar's flow sensor is very poor. In particular, oscillator frequency shows almost no change for flow rate variation from 0 to 2,000 cm.sup.3 /min., and even after 2,000 cm.sup.3 /min., the sensitivity is very low, i.e. a fractional frequency change, .DELTA.f/f, of less than 8.times.10.sup.-8 per cm.sup.3 /min. change in flow rate. Also, the device consumes relatively large heater power (greater than 1 W).
Another approach using SAW as a flow sensor is the patent by Brace and Sanfelippo, U.S. Pat. No. 4,726,225, issued Feb. 23, 1988. In this device, Brace and Sanfelippo do not use a separate heater. Instead they convert a fraction of the r.f. energy in the oscillator loop to heat. This device has the following disadvantages: (1) the power needed to heat the delay line has to be supplied by the r.f. amplifier which requires the use of an amplifier with large power output capability, thus making the amplifier expensive; and (2) heating takes place only in the region of the acoustic absorbers. Thus heating is only produced at certain localized regions of the substrate. This can cause nonuniform substrate temperature distribution, unless special arrangements are made to uniformly distribute the heat.
Theoretical analysis of flow sensor operation shows that to obtain a sensor with high sensitivity and fast response the following are needed: (1) efficient heating of SAW propagation path, (2) good thermal isolation between substrate and ambient, (3) a device with small mass, and (4) a substrate with a large temperature coefficient of delay.
Accordingly, the present invention provides a number of improved designs to meet the above goals. More particularly, the SAW flow sensor devices of the present invention locate the heater directly in the propagation path of the delay line. This results in efficiently heating the delay line and further no separate area for the heater is needed. Also there is obtained a very sensitive device. For example, in one device constructed in accordance with the invention, the value of .DELTA.f/f for 1 cm.sup.3 /min. change in flow rate is greater than 1.9.times.10.sup.-6. This is achieved with a heater power of less than 85 mW.